Definition of Ultraviolet radiation

Ultraviolet radiation: Invisible rays that are part of the
energy that comes from the sun. Ultraviolet radiation can burn the
skin and cause skin cancer.

Ultraviolet radiation is made up of three types of rays --
ultraviolet A, ultraviolet B, and ultraviolet C. Although ultraviolet
C is the most dangerous type of ultraviolet light in terms of its
potential to harm life on earth, it cannot penetrate earth's
protective ozone layer. Therefore, it poses no threat to human,
animal or plant life on earth.

Ultraviolet A and ultraviolet B, on the other hand, do penetrate
the ozone layer in attenuated form and reach the surface of the
planet. Because ultraviolet A is weaker than ultraviolet B,
scientists long blamed ultraviolet B as the sole culprit in causing
skin cancer in persons with a history of sunburn and repeated
overexposure to ultraviolet radiation. Recent research, however, has
also implicated ultraviolet A as a possible cause of skin cancer.

Ultraviolet B rays are more likely than ultraviolet A rays to
cause sunburn, but ultraviolet A passes further into the skin.
Scientists have long thought that ultraviolet B can cause melanoma
and other types of skin cancer. They now think that ultraviolet A may
add to skin damage that can lead to cancer. For this reason, skin
specialists recommend that people use sunscreens that block both
kinds of radiation.

In addition to natural light from the sun, artificial
light from tanning lamps contains ultraviolet A and ultraviolet B.
Electric arc lamps can also generate ultraviolet light to heat
furnaces for melting and to enable motion-picture projectors to show
movies.

Though ultraviolet light can damage health, it can also maintain
or improve health. When ultraviolet light strikes human skin, it
triggers the production of vitamin D, which promotes the growth of
bones and teeth.